It is known that an internal combustion engine of a motor vehicle generally includes a fuel injection system having a high pressure fuel pump, which delivers fuel at high pressure to a fuel rail, and a plurality of fuel injectors in fluid communication with the fuel rail. Each injector is provided for injecting metered quantities of fuel inside a corresponding combustion chamber of the engine. Conventionally, each fuel injector performs a plurality of injection pulses per engine cycle, according to a multi-injection pattern. This multi-injection pattern usually includes a main injection, which is executed to generate torque at the crankshaft, and several smaller injections, which may be executed before the main injection (e.g. pilot-injections and pre-injections) and/or after the main injection (e.g. after-injections and post-injections). Each of these small injection pulses is made to inject into the combustion chamber a small quantity of fuel, typically lower than 2.5 mm3 (for example 1 mm3), with the aim of reducing polluting emissions and/or combustion noise of the internal combustion engine.
The fuel injectors are essentially embodied as electromechanical valves having a needle, which is normally biased in a closed position by a spring, and an electro-magnetic actuator (e.g. solenoid), which moves the needle towards an open position in response of an energizing electrical current. The energizing electrical current is provided by an electronic control unit, which is generally configured to determine the fuel quantity to be injected by each single injection pulse, to calculate the duration of the energizing electrical current (i.e. the energizing time) needed for injecting the desired fuel quantity, and finally to energize the fuel injector accordingly.
However, it may happen that the fuel quantity actually injected during an injection pulse is different from the desired one. This undesirable condition may be caused by several factors, including drift of the injection characteristics and production spread of the fuel injectors. In particular, the correlation between the electrical command and the injector needle displacement can be affected by variables that are difficult to control during the injectors manufacturing, such as magnetic permeability drift of the actuator, tolerance of the needle spring coefficient, aging effect, and temperature dependency. Therefore, it is very likely that two fuel injectors (even of the same production slot) behave differently in response of the same electrical command.
As a result of these factors, for a given energizing time and a given fuel rail pressure, the fuel quantity actually injected into the combustion chambers of an internal combustion engine may be different injector-by-injector and/or vary with the aging of the injection system. This problem is particularly critical for the small injection pulses, whose accuracy and repetitiveness is important to achieve the expected improvements in terms of polluting emission and combustion noise.
To solve this drawback, while the internal combustion engine is running under cut-off conditions, the electronic control unit is conventionally configured to perform from time to time a procedure aimed to measure the actual fuel quantity which is injected by each fuel injector. According to the known solutions, the actually injected fuel quantity may be estimated on the basis of input signals deriving from different kinds of sensors such as knock sensors or on the basis of the crankshaft wheel signal.
A drawback of these prior solutions lies in the fact that such fuel quantity estimation is indirect and that the signals involved, for example the crankshaft wheel signal or other signals, are easily affected by noise and all sorts of disturbances coming from external environment such as rough roads, electric loads or other external or internal conditions, so that the resulting estimation may be not always reliable. Another drawback is that some of these known solutions cannot be performed during the execution of the so-called stop-start running strategies.
The stop-start running strategies are strategies that provide for disengaging the clutch and shutting off the engine when the motor vehicle is coasting, thereby saving fuel and reducing pollutant emissions. Under these circumstances, since the clutch is disengaged, some of the sensors involved in the conventional estimation of the fuel injected quantity cannot be used.
Still another drawback of the known solutions is that they are not able to measure the Start Of Injection (SOI). The SOI is a parameter that represents the instant when the injection pulse starts and is usually expressed in terms of an angular position of the engine crankshaft.
The SOI ideally coincides with the instant when the electronic control unit applies the energizing current to the fuel injector. However, due to fuel injector configurations (particularly solenoid injectors), there is always a certain delay between the application of the energizing current and the actual opening of the fuel injector. This delay is not the same for all the fuel injectors but is affected by the same factors that also affect the fuel injected quantity, such as for example the magnetic permeability drift of the actuator, the tolerance of the needle spring coefficient, aging effect, and temperature dependency. As a consequence, it may happen that two fuel injectors of the same kind (e.g. of the same production slot) open at different instants, even if the energizing current is applied at the very same time.